Method and apparatus for adaptive automatic gain control

ABSTRACT

Systems and methods for dynamically adjusting the gain in a receiver front end to have a desired amount of headroom, based upon a measurement of the signal to noise ratio (SNR) of the output of a digital to analog converter and the amount of degradation to the SNR due to previous adjustments to the gain.

RELATED APPLICATIONS

This application is a continuation of Ser. No. 15/303,355 filed on Oct.11, 2016, which is the nationalization of PCT applicationPCT/US2015/025252, which claims priority to U.S. application Ser. No.14/271,060 filed on May 6, 2014, now U.S. Pat. No. 9,124,234, whichmakes reference to and claims priority to U.S. Provisional ApplicationSer. No. 61/978,642 filed on Apr. 11, 2014. Each of the above statedapplications is hereby incorporated herein by reference in its entirety.

TECHNICAL FIELD

The disclosed method and apparatus relate to broadband receivers, andmore particularly to systems for efficiently receiving broadband signalswith large dynamic range.

BACKGROUND

Designers of broadband receivers face several challenges today. One suchchallenge is presented by the very nature of broadband receivers. Thatis, in many of the communications systems of today, receivers arerequired to digitize incoming signals over a broad frequency range.These signals may have a very large dynamic range (i.e., power levelscan vary greatly). For the purpose of this discussion, the term “powerlevel” is used broadly to refer to either the amount of power in aparticular frequency band (i.e., power density) or the amount of powerover the entire band used by the particular received signal (i.e., totalsignal power). In addition, a “signal” is defined as the energy within afrequency band that is used to represent information (i.e., “content”)being communicated from a transmission source to the receiver.Accordingly, a receiver may receive several signals, some such signalsfrom the same transmission source, each such signal using a differentfrequency band, and some such signals from different transmissionsources using different frequency bands.

FIG. 1 is a simplified block diagram of a typical receiver 100. A lownoise amplifier (LNA) 102 initially receives the incoming signals. TheLNA 102 amplifies the signals. The output from the LNA 102 is thecoupled to an input of an automatic gain control (AGC) circuit 103. TheAGC circuit 103 attempts to ensure that the power of the incoming signalremains essentially constant for incoming signals of varying powerlevels. It is common today for such AGC circuits to drive the incomingsignal to the highest level possible without causing distortion due toclipping of the signal.

The output of the ACG circuit 103 is coupled to the input of a set ofanalog filters 104. These analog filters 104 ensure that energy outsidethe desired frequency band is removed before further processing of thesignals. The filtered signals are then output from the analog filters104 to an analog to digital converter (ADC) 106. The ADC 106 takes thereceived filtered analog signals and generates a digital representationof these received signals. The digital representation of the analoginput takes the form of a series of digital values that each indicatethe amplitude of the analog signal at a particular point in time. Thecombination of the LNA 102, the AGC circuit 103, the analog filters 104and the ADC 106 comprise the receiver front end 109.

The digital output from the ADC 106 is then coupled to a Receive (RX)processor 108. The RX Processor 108 performs whatever processing isdesired for the received signals. For example, the RX Processor 108might process the signals to extract viewable video content and preparethat content for display on a monitor (not shown).

One problem that arises comes from the fact that unexpected bursts ofinterference can occur that will drive the output of the AGC to a levelthat will exceed the capability of the ADC. That is, the output of theADC will be compressed, since the ADC will hit full scale before thesignal at the input of the ADC has reached its highest level.Accordingly, the ADC will output the same erroneous value (i.e., thefull scale value) for each input above that level that initially reachesthe full scale value. This is commonly referred to as “clipping” andcauses errors in data transmission. For example in networks that conformto the well-known Multimedia over Coax Alliance (MoCA) standard, a burstinterference that drives the ADC into clipping will result in datapacket errors. Such a burst interference may be caused by adjacentchannel activity or off-air interference from cell or LTE (Long TermEvolution) phones.

The AGC is designed to drive the input to the ADC 106 as close to fullscale as possible. In some cases, headroom is designed into the ACGcircuit 103 to ensure that there is some safety margin (i.e., a“backoff” or “headroom”) between the full scale value at the output ofthe ADC 106 and the value that is output by the strongest receivedsignal. For most signals, the peak power exceeds the average power. Theminimum headroom needs to be sufficient to accommodate the peak power.Typically, a fixed amount of backoff is provided to deal with the worstcase. In addition, the backoff accounts for unit to unit variability,temperature variations and other factors. Therefore, the AGC providesless gain than is desirable in some cases, but more gain than isnecessary in other cases. If the selection of what is “worst case” ismade too aggressively, then it is more likely that from time to time, aninterfering signal will cause clipping in the ADC 106. However, if amore conservative approach is taken, the likelihood is that for themajority of the time, the front end will not be providing as much gainas would otherwise be desired.

In some cases, this can be dealt with by adjusting the modulation thatis used and thus increasing the signal to noise ratio margin and soreducing the impact when interference is present. However, in manysystems in use today, it is not possible to control the modulationsufficient rapidly to account for short bursts of interference.Furthermore, in some cases, the adjustments to the modulation are madebased on an analysis of the signals that are made at discrete times.Interference might not be present during those discrete times.Therefore, the modulation will not be adjusted to account for theinterference.

Further complicating the matter, in some cases receiver front endcircuits use a variable power supply. In such cases, the amount of gainthat is appropriate for one power supply output level will not beappropriate for another power supply output level.

Accordingly, there is presently a need for an receiver front end thatcan receive signals with a very large dynamic range and digitize themwithout compressing the large signals, while still maintaining a highresolution for weaker signals in the face of intermittent bursts ofnoise.

SUMMARY

Various embodiments of the disclosed method and apparatus for receivingbroadband content are presented. Some of these embodiments are directedtoward systems and methods for establishing the proper level of gainadjustment within the front end of a receiver.

In accordance with the disclosed method and apparatus, a measure of thesignal to noise ratio (SNR) is made. Alternatively, other similarqualitative measurements can be made in place of the SNR measurement. Ifthe measured SNR is above a predetermined minimum value, then the gainin the AGC is reduced. Another measure of the SNR is then made. If theSNR remains above the minimum value, then the difference between the SNRprior to the gain adjustment and the SNR after the gain adjustment iscalculated. This difference is referred to as the “delta SNR”. If thedelta SNR is below an allowable SNR degradation value, then the gain isagain reduced by an incremental amount. This process continues until theSNR either drops below the minimum acceptable value or the delta SNRexceeds the acceptable amount.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed method and apparatus, in accordance with one or morevarious embodiments, is described with reference to the followingfigures. The drawings are provided for purposes of illustration only andmerely depict examples of some embodiments of the disclosed method andapparatus. These drawings are provided to facilitate the reader'sunderstanding of the disclosed method and apparatus. They should not beconsidered to limit the breadth, scope, or applicability of the claimedinvention. It should be noted that for clarity and ease of illustrationthese drawings are not necessarily made to scale.

FIG. 1 is a simplified block diagram of a typical receiver.

FIG. 2 is a simplified block diagram of a receiver in accordance withone embodiment of the disclosed method and apparatus.

FIG. 3 is a simplified block diagram of the AGC Control Module inaccordance with one embodiment of the presently disclosed method andapparatus.

FIG. 4 is a flow diagram of the functions performed by the combinationof the AGC Control Module and the AGC module of one embodiment of thepresently disclosed method and apparatus.

FIG. 5 illustrates the relationship between various noise components ofthe input to the ADC.

FIG. 6 illustrates the relationship between the headroom, the signallevel, and the noise components with increased headroom in accordancewith one embodiment of the presently disclosed method and apparatus.

FIG. 7 illustrates the advantages in a receiver's ability to handleburst interference in accordance with the disclosed method andapparatus.

The figures are not intended to be exhaustive or to limit the claimedinvention to the precise form disclosed. It should be understood thatthe disclosed method and apparatus can be practiced with modificationand alteration, and that the invention should be limited only by theclaims and the equivalents thereof.

DETAILED DESCRIPTION

In one example of the disclosed method and apparatus, a broadbandreceiver uses an automatic gain control (AGC) control module to controlthe gain of an AGC module within the front end of the receiver.

FIG. 2 is a simplified block diagram of a receiver 200 comprising areceiver (RX) processor 208 and a receiver front end 209. The receiverfront end 209 comprises a low noise amplifier (LNA) 202, an AGC module203, an analog filter module 204, an analog to digital converter (ADC)206 and an AGC Control Module 210. An input signal 201 is coupled to theinput of the LNA 202. The output of the LNA 202 is coupled to the inputof the AGC module 203. The AGC module 203 provides variable gain to thesignal output from the LNA 202. The gain controlled output from the AGCmodule 203 is then coupled to the input of the analog filter module 204.The analog filter module 204 removes signals that are outside of thefrequency range of interest. The output from the analog filter module204 is then coupled to the input of the ADC 206. The ADC 206 digitizesthe analog signal output from the filter module 204. The digitizedsignal can then be processed by the Rx processor 208. The RX processor208 provides decoding, demodulating, video processing, etc. It will beunderstood by those skilled in the art that the type of processing thatthe RX processor 208 performs will depend upon the type of receiver 200.

The AGC Control Module 210 receives information from the ADC 206. Thereceived information allows the AGC Control Module 210 to make informeddecisions regarding the amount of gain to be added to input signal 201to ensure that the ADC 206 is used most efficiently and effectively. Inaccordance with one embodiment of the disclosed method and apparatus,the information received from the ADC 206 is sufficient to allow the AGCcontrol module 210 to determine the signal to noise ratio (SNR) of thereceived signal.

FIG. 3 is a simplified block diagram of the AGC Control Module 210. FIG.4 is a flow diagram of the functions performed by the combination of theAGC Control Module 210 and the AGC module 203. The AGC Control Module210 includes a signal to noise ratio (SNR) measurement module 302 and again processor 304. The SNR measurement module 302 receives informationfrom the ADC 206 from which the SNR measurement module 302 determinesthe SNR of the received signal (STEP 402). Alternatively, the RXprocessor 208 provides the SNR of the received signal 201 directly tothe AGC Control Module 210. In that case, the SNR measurement module 302may not be required. Accordingly, the input to the gain processor 304 iscoupled to the RX processor 208. In either case, the gain processor 304receives a measure of the SNR of the input signal 201. The gainprocessor 304 determines whether the SNR value is greater than apredetermined minimum SNR value (STEP 404). The predetermined minimumSNR value can be determined based upon the minimum value needed todecode information encoded on the input signal 201 with the required (ordesired) accuracy.

If the SNR is not above the minimum required SNR, then in accordancewith one embodiment of the disclosed method and apparatus, a check ismade as to whether there is sufficient headroom to allow the gain of theAGC module 203 to be increased. In one embodiment of the presentlydisclosed method and apparatus, the AGC module 203 provides informationto the ADC 206 from which this determination can be made. In one suchembodiment, the determination is made based on predetermined knowledgeof the peak-to-average ratio for signals that are likely to be received.The headroom should be at least some margin greater than thepeak-to-average ratio. Alternatively, the peak-to-average ratio can beassumed to be in line with a Gaussian distribution of values. The amountof headroom can then be selected to have some margin over thepeak-to-average ratio. In yet another embodiment, the output of the ADC206 is statistically analyzed to determine the likelihood that clippingwill occur at the gain value. This analysis is then used to determinethe amount of headroom desired.

If there is sufficient headroom, then the gain is increased (STEP 416).However, if there is not sufficient headroom, then a further increase inthe gain will result in further distortion and a degradation in the SNR.That is, it should be noted that it is possible that the SNR is less thethen minimum (STEP 404) due to clipping of a relatively large inputsignal. Clipping will increase the amount of distortion that existswithin the noise component ND_(ADC) which is discussed further belowwith regard to FIGS. 5 and 6. Additional noise in the noise componentND_(ADC) will drive the SNR down. Therefore, in one embodiment of thedisclosed method and apparatus, if there is insufficient headroom (STEP414), then the process will reduce the gain (STEP 412).

If the SNR is above the minimum required SNR, the gain processor 304stores the value (STEP 406). If this is the first time through theprocess, then there will not be a previously saved SNR value. (STEP408). If that is the case, then the gain processor 304 sends a commandto the AGC module 203 to the AGC module 203 to adjust the gain down(STEP 412). In one embodiment, this downward adjustment is a relativelysmall incremental step.

If there was a previously saved value (STEP 408), then the differencebetween the saved value and the current value is calculated. Thisdifference is referred to as the “delta SNR”. If the delta SNR issmaller than a “maximum delta SNR” value (STEP 410), then the gainprocessor 304 sends a command to the AGC module 203 to adjust the gaindown (STEP 412) in order to establish the SNR at a predetermined level.If not, then in one embodiment of the disclosed method and apparatus,the gain processor 304 sends a command to the AGC module 203 to adjustthe gain up (STEP 414) to establish the SNR at the predetermined level.The process will then repeat continuously as shown by the arrow back toSTEP 402 from either STEP 412 or STEP 416.

The delta SNR is a way to measure the amount of influence the noisecomponent ND_(ADC) 506 generated within the ADC 206 has one the SNR.This will be discussed further below with regard to FIG. 5 and FIG. 6.

In an alternative embodiment, the gain remains unchanged if the deltaSNR is greater than the maximum delta SNR value. In another alternativeembodiment, adjustments to the gain may cease if the output from the ADC206 falls outside a desired range (i.e., either the amount of headroomfalls below a desired minimum or the level of the ADC 206 output fallsbelow a desired minimum).

It can be seen that this process will result in the gain beingestablished at the point at which the SNR is very near the minimumrequired SNR for accurate demodulation (i.e., the SNR will beestablished at a predetermined level). In accordance with oneembodiment, the adjustments to increase the gain are smaller than theadjustments to reduce the gain. In an alternative embodiment, theadjustments are the same size for both reducing and increasing the gain.In yet another embodiment, the adjustments to increase the gain arelarger than the adjustments to reduce the gain. In yet anotherembodiment, the adjustments in the gain are non-monotonic to reduce thelikelihood of divergence, increase the rate of convergence or reduce thelikelihood or magnitude of oscillations in the gain. In yet anotherembodiment, the rate at which the gain converges to the desired level isincreased by having larger gain adjustment steps during an acquisitionmode and smaller gain adjustment steps during a maintenance mode. In yetanother embodiment, the size of the adjustment can be based on thenumber of consecutive times the questions in STEP 404 and STEP 410return the same answer. The more times the same answer is returned, thelarger the adjustment. Once the answer to one of the questions changes,the size of the adjustment decreases. In yet another embodiment,hysteresis can be provided by changing the minimum in STEP 404 andchanging the maximum used in STEP 410. That is, increasing the minimumused in STEP 404 for the next measurement if the last answer to thequestion in STEP 404 has changed from NO to YES. This ensures that smallchanges in the SNR do not cause the gain to be adjusted. Likewise, theminimum used in STEP 404 for the next measurement can be decreased ifthe last answer to the question in STEP 404 has changed from YES to NO.In one such embodiment, no adjustment to the gain will be made unlessthe difference between the SNR and the minimum is greater than apredetermined amount. The decision made in STEP 410 can be likewiseadapted. In yet another embodiment, a check is made to ensure that thereis sufficient headroom between the maximum amplitude input to the ADC206 and the amplitude that will cause the ADC 206 to output a full scalevalue.

FIG. 5 illustrates the relationship between various noise components ofthe input to the ADC 206. The incoming signal 201 is shown to have asignal level 502. The ADC full scale value is shown at a level 504.Three noise components are shown. The first is the noise componentND_(ADC) 506 generated within the ADC 206. This noise component 506includes the noise that is due to distortion that occurs in the ADC 206.The second component of noise ND_(Sig) 508 is that noise which isreceived at the input of the LNA 202 together with the signal 201.ND_(Sig) 508 includes any distortion that occurs to the incoming signalprior to reception at the input to the LNA 202. The third component ofnoise ND_(RFFE) 510 is the noise that is generated within the LNA 202(i.e., RF front end noise and distortion). The sum of these three noiseand distortion components is shown as ND_(tot1) 512. It will beunderstood that the difference between ND_(tot1) 512 and the incomingsignal level 502 is the SNR₁ 514, the SNR of the incoming signal 201 tothe total noise ND_(tot1) 512.

Headroom 516 is defined as the difference between the signal level 502and the full scale value 504 of the ADC 206. In the case in which afixed headroom 516 is maintained, the signal level 502 will becontrolled by the AGC module 203 to maintain the fixed headroom 516regardless of the value of the noise components or the SNR.

FIG. 6 illustrates the relationship between the headroom 616, the signallevel 602, and the noise components ND_(ADC) 606, ND_(Sig) 608,ND_(RFFE) 610, ND_(tot1) 612 of a receiver front end 209 in accordancewith one embodiment of the presently disclosed method and apparatus. Byreducing the gain of the LNA 202, the levels ND_(Sig) 608, ND_(RFFE)610, N_(tot1) 612 will each be reduced to some extent. As the AGCControl Module 210 performs its above described functions, the amount ofheadroom 616 will increase while the SNR₂ remains essentially equal toSNR₁. Headroom 616 is defined as the difference between the signal level602 and the full scale value 604 of the ADC 206.

This will be the case as long as ND_(Sig) 608 +ND_(RFFE) 610 issubstantially greater than ND_(ADC) 606. It should be noted that thedepiction in FIG. 6 may not show the end result of the process that isillustrated in FIG. 4. That is, FIG. 6 may not depict SNR₂ 614 being atthe minimum SNR value as determined in STEP 404. It can be seen byreference to FIG. 2 that since the noise components ND_(Sig) 608 andND_(RFFE) 610 are present at the input to the AGC module 203, they willeach be amplified or attenuated by whatever amplification or attenuationis provided by the AGC module 203. However, the noise component ND_(ADC)606 is generated in the ADC 206. Therefore, this component is notpresent at the input of the AGC module 203. Accordingly, changes in theamount of gain or attenuation provided by the AGC module 203 will nothave any effect on ND_(ADC) 606. Therefore, the result of lowering thegain of the AGC module when ND_(ADC) 606 is substantially lower thanND_(Sig) 608 +ND_(RFFE) 610 is to lower ND_(tot1) 612 by an amount thatis essentially the same as the amount as the signal level 602. Thus,SNR₂ 614 remains essentially the same as SNR₁ 514.

However, it can be seen that there is a point at which reducing the gainfurther will cause the relative significance of ND_(ADC) 606 to be suchthat a reduction in the gain (and thus the signal level 602) will causethe SNR to go down. One way to measure the point at which that affectbecomes significant is to measure the delta SNR (i.e., the amount ofchange in SNR with a change in gain). It is for this reason that thedelta SNR is monitored. In addition, the delta SNR is used to determinewhether the gain has been reduced too much (see STEP 410 of FIG. 4). Inaccordance with one embodiment of the presently disclosed method andapparatus, the gain should not be reduced below the point at whichND_(ADC) 606 starts to dominate be a significant portion of ND_(tot1)612. That is, ensuring the delta SNR is below a maximum value willensure that the SNR does not drop significantly as a consequence of areduction in the gain provided by the AGC module 203.

FIG. 7 illustrates the advantages achieved by the disclosed method andapparatus in terms of a receiver's ability to handle the burstinterference. A first curve 701 shows the relationship between thesignal level at the input to the ADC 206 and varying receive signallevels in a system in which the headroom is maintained at a fixed value.In such a system, the AGC module 203 will attenuate signals that come inat relatively high levels. When the received signal level gets lower,the AGC module 203 provides gain to ensure that the signal level at theinput of the ADC 206 remains constant (i.e., constant headroom). Thisresults in waste in the dynamic range of the ADC 206, as will be seen inmore detail below.

A dashed line 702 illustrates the level required at the input to the ADC206 to achieve an acceptable error rate for the received data. That is,for each receive level, when the level at the input to the ADC 206 is atleast at the level indicated by the dashed line 702, the error rate willbe acceptable. If the level at the input to the ADC 206 goes below thatlevel, then the noise component ND_(ADC) 606 will become sufficientlysignificant to effect SNR and thus increase the error rate. It should benoted that as the receive signal level goes down (moving from left toright across FIG. 7) the SNR will go down one dB for each one dB drop inreceive signal level. That is, the noise components ND_(RFFE) 610 andND_(Sig) 608 will remain constant while the signal level 502 decreases(See FIG. 5 and FIG. 6). Increases in the gain of the signal do notalter the SNR, just as decreases in the gain will not alter the SNR.That is, as noted in FIG. 5 and FIG. 6, as the gain is reduced (orincreased), the amount of noise contributed by ND_(RFFE) 610 andND_(Sig) 608 will also go down (or up), since these components of noiseare directly amplified or attenuated by the AGC module 203. As notedabove, as long as the sum of noise components ND_(RFFE) 610 and ND_(Sig)608 is much greater than the noise component ND_(ADC) 606 (which is notamplified by the AGC module 203), changes in the gain of the AGC module203 will not affect the SNR.

In order to ensure that the data can be demodulated with an acceptableerror rate, the modulation density of the transmitted signal is reducedsufficiently to ensure that the SNR of the received signal is sufficientto allow demodulation at the desired error rate. Modulation density isdefined as the density of the modulation constellation used to representthe data. For example, data can be modulated using quadrature amplitudemodulation (QAM) having a constellation with 1024 points. Alternatively,the data can be modulated with a less dense QAM constellation (e.g., 512points). In accordance with one embodiment of the presently disclosedmethod and apparatus, an SNR of approximately 30.3 dB is required toachieve the desired error rate when the data is modulated with QAM 1024.For a modulation using half the number of constellation points (i.e.,QAM 512), the SNR required drops by 3 dB. The SNR will drop anadditional 3 dB each time the number of constellation points is reducedby one half.

In one embodiment in which the received signal is an OFDM modulatedsignal with bit loading (such as used in MoCA), a relatively largenumber of carriers are each modulated with a modulation density that isindependent of the other carriers. Each carrier is modulated dependingupon the conditions of that carrier. The adjustment to the modulation isaveraged over all of the carriers. Accordingly, the amount of the changein the overall modulation density can be adjusted in relatively finesteps. This is because each carrier can change independently, having arelatively small impact on the modulation density of the overall signal.Accordingly, the change to the overall SNR required can be made inrelatively small steps. That is, the amount of power in any one receivedcarrier will be a relatively small percentage of the overall total powerof the receive signal. Accordingly, small changes in the receive signallevel can be offset by small changes in the modulation density of theoverall signal by changing the modulation density of a small number ofthe carriers. The small changes in the modulation density translate tosmall changes in the SNR requirement represented by dashed line 702.Thus, the dashed line 702 representing the required SNR is a shown as acontinuous line that drops smoothly at a rate of 1 dB for each dB dropin the level of the received signal (i.e., the steps are small enough toshow the line as continuous rather than as a series of stair steps).

The dashed line 702 starts at an “Rx knee” 704. The Rx knee occurs atthe point 704 at which the SNR drops below the level required to be ableto accurately demodulate data at the required error rate. At that point,further increases in gain by the AGC module 203 will have relativelysmall impact on the SNR, since the sum of noise components ND_(RFFE) 610and ND_(Sig) 608 is much greater than the noise component ND_(ADC) 606.Therefore, the SNR requirement must be reduced by reducing themodulation density if the desired error rate is to be maintained. Thedashed line 702 shows the curve of the reduction in the required SNRresulting from a reduction in the modulation density.

A second curve 703 shows the relationship between the level of the ADCinput and the receive signal level for a system in which the SNR marginis maintained constant. If the SNR margin is maintained relativelyconstant, rather than maintaining the headroom constant for variouslevels at the input of the ADC 206, dynamic range that is otherwisewasted is reclaimed. That is, the ADC 206 can be operated closer to thecenter of its range, allowing more flexibility to handle deviations inreceive signal level. That is, as long as the noise component ND_(ADC)606 remains well below the level of the noise components ND_(RFFE) 610and ND_(Sig) 608, the amount of gain provided by the AGC module 203 willnot affect the SNR. Therefore, adding more gain reduces the dynamicrange of the ADC 206 without providing a significant benefit.

It should be noted that the ADC input level for curve 703 is lower(i.e., the gain applied by the AGC module 203 is less) even atrelatively high receive signal levels. Thus, greater dynamic range isprovided in the ADC 206. This is possible, since the SNR is acceptableat such lower levels. This reduction in the input level to the ADC 206increases the dynamic range of the ADC 206. However, the advantage iseven greater as the level of the receive signal goes down. This can beseen by the divergence of line 701 from line 703, starting at the point705 and continuing up to the point 707 at which the gain of the AGCmodule 203 is at the maximum possible. At that point, the differencebetween the level shown in line 701 and line 703 remains constant. Itshould be noted that how much gain is provided above that necessary toensure the SNR is sufficient to allow accurate demodulation at thehighest modulation rate possible can vary depending upon implementationdetails within the particular embodiment of the disclosed method andapparatus.

At point 705, the amount of gain provided by the AGC module 203 can bereduced while maintaining a constant SNR margin 711. In contrast, if afixed amount of headroom is maintained, the AGC module 203 continues toincrease the amount of gain to maintain a constant ADC input level(i.e., maintain a fixed amount of headroom). This continues until theAGC module 203 reaches its maximum gain at the point 707. Only then doesthe level at the input of the ADC 206 begin to go down. It can be seenthat in the case of a system operating in accordance with line 703,having a lower ADC input level allows for greater dynamic range. Thatis, since the ADC input level is lower, sudden bursts that cause theinput level to rise will be less likely to be clipped.

When the receive signal level is −64 dBm, the difference between thedynamic range of the system operating with fixed headroom and the systemoperating with only enough gain to ensure that the maximum SNR margin isachieved reaches a maximum 709. It should be noted that at some point, aminimum level might be desired at the ADC 206 to keep the ADC input nearthe center of the dynamic range of the ADC 206. However, that wouldassume that there is sufficient gain available in the AGC module 203 toallow the signal to be boosted.

While various embodiments of the disclosed method and apparatus havebeen described above, it should be understood that they have beenpresented by way of example only, and should not limit the claimedinvention. Likewise, the various diagrams may depict an examplearchitectural or other configuration for the disclosed method andapparatus. This is done to aid in understanding the features andfunctionality that can be included in the disclosed method andapparatus. The claimed invention is not restricted to the illustratedexample architectures or configurations, rather the desired features canbe implemented using a variety of alternative architectures andconfigurations. Indeed, it will be apparent to one of skill in the arthow alternative functional, logical or physical partitioning andconfigurations can be implemented to implement the desired features ofthe disclosed method and apparatus. Also, a multitude of differentconstituent module names other than those depicted herein can be appliedto the various partitions. Additionally, with regard to flow diagrams,operational descriptions and method claims, the order in which the stepsare presented herein shall not mandate that various embodiments beimplemented to perform the recited functionality in the same orderunless the context dictates otherwise.

Although the disclosed method and apparatus is described above in termsof various exemplary embodiments and implementations, it should beunderstood that the various features, aspects and functionalitydescribed in one or more of the individual embodiments are not limitedin their applicability to the particular embodiment with which they aredescribed. Thus, the breadth and scope of the claimed invention shouldnot be limited by any of the above-described exemplary embodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

A group of items linked with the conjunction “and” should not be read asrequiring that each and every one of those items be present in thegrouping, but rather should be read as “and/or” unless expressly statedotherwise. Similarly, a group of items linked with the conjunction “or”should not be read as requiring mutual exclusivity among that group, butrather should also be read as “and/or” unless expressly statedotherwise. Furthermore, although items, elements or components of thedisclosed method and apparatus may be described or claimed in thesingular, the plural is contemplated to be within the scope thereofunless limitation to the singular is explicitly stated.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all configured in acommon package. Indeed, any or all of the various components of amodule, whether control logic or other components, can be combined in asingle package or separately maintained and can further be distributedin multiple groupings or packages or across multiple locations.

Additionally, the various embodiments set forth herein are described interms of exemplary block diagrams, flow charts and other illustrations.As will become apparent to one of ordinary skill in the art afterreading this document, the illustrated embodiments and their variousalternatives can be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

What is claimed is:
 1. A receiver front end comprising: an automaticgain control (AGC) module having an input and an output; an analog todigital converter (ADC) having an input and an output, the input coupledto the output of the AGC module; and an AGC control module having aninput for receiving information regarding the signal output from the ADCand an output for controlling adjustments to the gain of the AGC module;wherein the AGC control module controls the gain of the AGC module,determines signal to noise ratio (SNR) for the signal output from theADC, and determines a delta SNR level that is based on a change of theSNR resulting from a change in the gain of the AGC module.
 2. Thereceiver front end of claim 1, wherein: when the SNR is above a minimumlevel, then the AGC control module determines whether a delta SNR isabove a maximum level, and when the SNR is not above a minimum level,the AGC control module adjusts the gain up; and when the SNR is above aminimum level and the delta SNR is above a maximum level, the AGCcontrol module adjusts the gain up, otherwise, the AGC module adjuststhe gain down.
 3. The receiver front end of claim 2, wherein determiningwhether a delta SNR is above a maximum level includes: storing a firstSNR value; determining whether a previous SNR value has been stored; andwhen a previous SNR value has been stored, then determining the deltaSNR by calculating the absolute value of the difference between theprevious SNR value and the current SNR value.
 4. The receiver front endof claim 1, wherein the determining whether a delta SNR is above amaximum and adjusting the gain of the AGC are repeated.
 5. The receiverfront end of claim 1, wherein: when the SNR is above a minimum level,then the AGC control module determines whether a delta SNR is above amaximum level, and if the SNR is not above a minimum level, the AGCcontrol module determines whether there is sufficient headroom to adjustthe gain up, and if so, then adjusts the gain up; when the SNR is abovea minimum level and the delta SNR is above a maximum level, the AGCcontrol module determines whether there is sufficient headroom to adjustthe gain up and if so, then adjusts the gain up, otherwise, the AGCcontrol module adjusts the gain down.
 6. The receiver front end of claim5, wherein: when the SNR is not above a minimum level and there issufficient headroom, the AGC control module adjusts the gain up, and ifeither the SNR is above the minimum or there is insufficient headroom,the AGC control module adjusts the gain down.
 7. The receiver front endof claim 5, wherein the determining whether a delta SNR is above amaximum level, determining whether there is sufficient headroom toadjust the gain up, and adjusting the gain up or down are repeated. 8.The receiver front end of claim 5, wherein adjustments to increase thegain are equal in size to adjustments to reduce the gain.
 9. Thereceiver front end of claim 5, wherein adjustments to increase the gainare larger than adjustments to reduce the gain.
 10. The receiver frontend of claim 5, wherein adjustments to increase the gain are smallerthan adjustments to reduce the gain.
 11. The receiver front end of claim5, wherein gain adjustments during an acquisition mode are greater thangain adjustments during a maintenance mode.
 12. The receiver front endof claim 5, wherein the size of the gain adjustment is based on thenumber of consecutive times the gain has been adjusted in the samedirection.
 13. The receiver front end of claim 1, wherein the receiverfront end comprises a low noise amplifier (LNA), the output of the LNAbeing coupled to the input of the AGC module, and wherein the SNR is theratio of the desired signal level to the level of the noise that is dueto distortion that occurs in the ADC, noise that is received at theinput of the LNA together with a signal coupled to the input of the LNAand any distortion that occurs to the incoming signal prior to receptionat the input of the LNA and any noise generated within the LNA.
 14. Thereceiver front end of claim 1, comprising an analog filter modulecoupled between the output of the AGC module and the input of the ADC.15. A method for controlling the gain of a receiver front end,comprising: receiving an input signal; measuring the signal-to-noiseratio (SNR) of the input signal; determining a delta SNR based on achange in gain in an automatic gain control (AGC) module in the receiverfront end; adjusting the gain of an AGC module based on the SNR; andrepeating the gain adjusting until either the SNR is below a minimumdesired level or the delta SNR is above a minimum.
 16. The method ofclaim 15, wherein determining the delta SNR comprises making a firstmeasurement of an SNR before making an adjustment to the gain of the AGCmodule, making a second measurement of an SNR after making an adjustmentto the gain of the AGC module and determining the difference in the SNRbetween the first and second measurement.
 17. The method of claim 15,comprising increasing the gain of the AGC module when either the SNR isbelow the minimum desired level or the delta SNR level is above amaximum and there is sufficient headroom, and then repeating the method.18. The method of claim 15, wherein: when the SNR is above a minimumlevel, then the AGC control module determines whether a delta SNR isabove a maximum level, and when the SNR is not above a minimum level,the AGC control module adjusts the gain up; and when the SNR is above aminimum level and the delta SNR is above a maximum level, then the AGCcontrol module adjusts the gain up, otherwise, the AGC module adjuststhe gain down.
 19. The method of claim 15, wherein: when the SNR isabove a minimum level, then the AGC control module determines whether adelta SNR is above a maximum level, and when the SNR is not above aminimum level, the AGC control module determines whether there issufficient headroom to adjust the gain up, and when so, then adjusts thegain up; when the SNR is above a minimum level and the delta SNR isabove a maximum level, the AGC control module determines whether thereis sufficient headroom to adjust the gain up and when so, then adjuststhe gain up, otherwise, the AGC control module adjusts the gain down.20. A receiver front end, the receiver front end being operable to:receive an input signal; measure a signal to noise ratio (SNR) of theinput signal; adjusting the gain of an automatic gain control (AGC)module in the receiver front end based on available gain headroom;determine a delta SNR level that is based on a change of the SNRresulting from a change in the gain of the AGC module; and repeat thegain adjusting and delta SNR determining.